Touch-sensitive display screens are a well known component of many personal electronic devices. By noting and responding to a user's touch (usually with a finger or a stylus), these screens both gather user input and present the device's output in a unified way that is very appealing for many applications.
The first popular touch-sensitive display screens could only reliably note one touch location at a time. In the presence of multiple simultaneous touches, these display screens would become confused and unpredictable. Now, however, many devices incorporate screens that reliably track several simultaneous touches and can measure the total pressure exerted by the user.
One of the appealing aspects of touch-sensitive display screens is that they can, in some instances at least, replace both the keyboard and the pointing device (e.g., a mouse or trackball) found on the more traditional personal computer. This makes these screens especially useful for very small devices such as smart phones where a keyboard, if present at all, is too small to be optimal for ten-finger typing.
Naturally, the development of user-interface modalities for touch-sensitive screens has followed the uses pioneered by the fixed keyboard and mouse. People very easily transition from using a mouse to control a cursor on the display screen to simply touching the screen and dragging the cursor where it is needed.